1. Field of the Invention
The present invention relates to a high pressure discharge lamp such as a high pressure mercury lamp, a high pressure sodium lamp, a metal halide lamp, and a high pressure xenon lamp.
2. Description of the Related Art
A high pressure discharge lamp is widely used as a light source which can supply light with a high luminous intensity. In general, a high pressure discharge lamp includes a light transmissible arc tube which is filled with a rare gas, mercury, metal halide and the like, and a pair of electrodes provided in the arc tube. FIG. 13 shows a cross section of a metal halide lamp which is one type of a conventional high pressure discharge lamp. This metal halide lamp includes a quartz arc tube (discharge tube) 1 which is filled with metal halide, mercury, and rare gas, and a first electrode 3a and a second electrode 3b. The arc tube 1 is sealed with pinches 2a and 2b which support the pair of electrodes 3a and 3b. Arc discharge is induced by voltage applied across the pair of electrodes 3a and 3b. The light emission of the filling materials due to the arc discharge is utilized for general lighting, lighting for an apparatus such as an overhead projector (OHP), or the like.
Hereinafter, a virtual straight line which connects one end of the first electrode 3a to one end of the second electrode 3b is sometimes referred to as an "electrode axis". When a lamp is lighting in such a manner that the electrode axis is set substantially horizontal, the lamp is said to be in horizontal lighting. Also, a lamp which is produced for the purpose of such use is referred to as a lamp of horizontal lighting type. On the other hand, when a lamp is lighting in such a manner that the electrode axis is set substantially vertical, the lamp is said to be in vertical lighting. Also, a lamp which is produced for the purpose of such use is referred to as a lamp of vertical lighting type.
The mass and ionization potential of a metal contained in the arc tube 1 varies depending on the kind of the metal. Based on the fact, for example in an Sc-Na type metal halide lamp, at the center of the arc, light is emitted by mercury. In the peripheral portion of the arc, light is emitted by Sc. In the further outer portion (in the outermost portion), light is emitted by Na. As a result, the emission spectrum is not uniform for various portions of the arc.
In addition, as is shown in FIG. 13, when the high pressure discharge lamp is in the horizontal lighting, the arc is curved due to buoyancy caused by the convective phenomenon of the filler gas. When the high pressure discharge lamp is in the vertical lighting, an upper portion of the arc is expanded, and a lower portion thereof is narrowed (pointed), so that the arc has the shape of teardrop. As a result of the curved or teardrop shape, the nonuniformity of emission spectrum for various portions of the arc is further increased.
When the arc of the high pressure discharge lamp is deformed due to the convection phenomenon of the filler gas as described above, it is difficult to design an optical system which uses the high pressure discharge lamp as a light source. Especially when the high pressure discharge lamp is used as a light source for a projection-type display, the deformation of the arc is magnified on the screen, whereby the quality of the projected image is degraded.
In addition, as is shown in FIG. 13, when the high pressure discharge lamp is in the horizontal lighting, the arc is curved due to buoyancy. As a result, the temperature of the upper portion of the arc tube 1 rises, so that the upper portion of the arc tube 1 is more thermally expanded, as compared with the lower portion of the arc tube 1. Such thermal expansion of the arc tube 1 causes the lifetime of the high pressure discharge lamp to be shortened.